Introducer sheaths are inserted percutaneously through the patient's skin, typically in the groin with their proximal end remaining outside the patient, and the distal end placed in an artery or vein. These sheaths include a deformable valve near their proximal end that is designed to seal a guide wire or catheter that is inserted through the sheath. The purpose of this seal is to prevent blood from leaking through the sheath. However, because the outside diameter of a guide wire is typically very much smaller than the outside diameter of the catheter that is inserted through the sheath, there is typically excessive blood leakage through the valve when the catheter is not in place.